With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data transmission performance over the copper wire local loop transmission lines that connect telephone central offices (COs) to customer premises (CPs). In conventional telephony networks, customer premises equipment (CPE) are coupled to CO switches over the above mentioned transmission lines, which are commonly known as “local loops,” “subscriber lines,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. In the art, the term “line” and “loop” are used interchangeably, both terms referring to the copper wire pair used in a typical telephone transmission line conductor. Historically, the public switched telephone network (PSTN) evolved with subscriber loops coupled to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. “Central office” or “CO” means any site where a subscriber loop couples to a telephony switching unit, such as a public switched telephone network (PSTN), a private branch exchange (PBX) telephony system, or any other location functionally coupling subscriber loops to a telephony network. Digital service provision to the CP is a more recent development. With it, the telephone network has evolved from a system capable of only carrying analog voice communications into a system that can simultaneously carry voice and digital data.
Historically, the POTS subscriber loop was designed with the functions needed to communicate analog voice-conversation signals and subscriber loop signaling. The CO switch uses subscriber loop signaling to notify the customer premises about events in the telephone network, while customer premises equipment (CPE) use subscriber loop signaling to inform the CO to perform actions for the customer. Some examples of subscriber loop signaling include: the CO switch signaling to the CPE that an incoming call has arrived by ringing the phone, the CPE (e.g., a telephone) signaling to the CO switch that the CPE is initiating a call by an on-hook to off-hook transition of the telephone handset, and the CPE signaling to the CO switch that a call should be connected to a location by transmitting the phone number of the location.
Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost migration to digital technologies. Subscriber loops capable of carrying digital signals are known as digital subscriber lines (DSLs). Various digital technologies provide customers with additional flexibility and enhanced services by utilizing frequency-division multiplexing and/or time-division multiplexing techniques to fully exploit the transmission capability of a subscriber loop. These newer DSL technologies provide digital service to the customer premises without significantly interfering with the existing plain old telephone service (POTS) equipment and wiring by utilizing portions of the available frequency spectrum not used by a POTS signal. These portions of the frequency spectrum are often referred to as “logical channels.” Logical channels within a subscriber line that carry digital signals are known as “DSL channels,” while logical channels within a subscriber line which carry POTS analog signals are known as “POTS channels.”
DSL technologies, such as but not limited to integrated services digital network (ISDN), high-bit-rate digital subscriber line (HDSL), HDSL2 and symmetric digital subscriber line (SDSL), utilize different frequencies of the available frequency spectrum and therefore do not coexist with a POTS signal, which typically utilizes the 0-4 kilo-hertz (KHz) portion of the available frequency spectrum. These DSL technologies accomplish this functionality by frequency-division multiplexing (FDM) a single data signal onto a logical channel above (at higher frequencies than) the 0 KHz to 4 KHz frequency range used by the analog POTS signals. Such multiplexing techniques and terminology are common to those skilled in the art, and are not described in detail herein.
Several variations of new multiple channel DSL technology exist, such as, but not limited to, Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), Very High Speed DSL (VDSL), Multiple Virtual Lines (MVL™) and Tripleplay™, with this group generally referred to as xDSL. Communications systems employing xDSL technology may multiplex a plurality of data signals and a single POTS signal onto a single subscriber line. An xDSL system employing frequency-division multiplexing would multiplex a plurality of data signals onto a corresponding plurality of logical channels, each logical channel utilizing a different portion of the available frequency spectrum. An xDSL system employing time-division multiplexing would multiplex a plurality of data signals onto a single logical channel with each different data signal allocated to a predefined portion of time in a predefined, repeating time period.
For example, an xDSL system employing time-division multiplexing of four data signals would subdivide a predefined time period into four sub-periods. Each one of the four data signals would be allocated to one of the four sub-periods. During the first sub-period, the first data signal would be communicated across the subscriber loop. During the second sub-period, the second data signal would be communicated. Likewise, the third and fourth data signals would be communicated during the third and fourth sub-periods, respectively. When the fourth sub-period has ended, the predefined time period repeats, and the first data signal is communicated during a new first sub-period. Thus, four individual data signals can be transmitted sequentially by allocating one of the signals to one of the four sub-periods.
FIG. 1 is a simplified illustrative block diagram of a portion of an existing telephony system 20 which includes a telephone company CO 22 coupled to a CP 24 via a single subscriber loop 26. Subscriber loop 26 may be any suitable connection for communicating electrical signals, but is typically a copper wire pair, as is well known in the art, that was originally designed to carry a 0-4 KHz analog voice channel (POTS signal). Located within the CO 22 is the CO telephony POTS switching unit 28 which communicates POTS signals with the telephone(s) 30 residing in CP 24 via the subscriber loop 26. In some instances, filter(s) 32 may be coupled between subscriber loop 26 and telephone(s) 30.
CO digital equipment 34 and low pass filter 36 may be added at the CO to facilitate transmission of digital data. Digital equipment 34 transmits and receives data signals over subscriber loop 26. When a copper wire pair is used for data signal transmission, the wire pair is often referred to as a digital subscriber loop (DSL).
Low pass filter 36 separates, or splits out, the POTS signal for delivery to POTS switching unit 28. Low pass filter 36 is designed to pass the 0-4 KHz analog POTS signal. In some applications, a POTS splitter (not shown) may be used. Such a POTS splitter may also include a high pass frequency filter designed to pass the data signals, which utilize the portion of the available frequency spectrum above 4 KHz, to the digital equipment 34. Thus, a POTS splitter may split off the data signal from the subscriber loop for delivery to digital device 38, thereby separating the data signal from the POTS analog signal. POTS splitter technology is well known in the art, and is therefore not described in detail herein.
Located within the CP 24 may be a plurality of digital equipment devices 40 which transmit and receive data signals over subscriber loop 26. Illustrative examples of digital equipment devices 40 include, but are not limited to, facsimile (FAX) machines, set top boxes, internet appliances, computers, personal computers (PCs) or the like. A digital device 38, such as a modem or the like, is coupled to or can be interfaced with the digital equipment devices 40 and subscriber loop 26. Digital device 38 may communicate with the plurality of digital equipment devices 40 via an ethernet 42, other local access network (LAN), or the like. Alternatively, digital device 38 may communicate with a single digital equipment device 40 via a cable (not shown). For convenience of illustration, digital device 38 is shown as being a separate device. However, digital device 38 may be incorporated into a digital equipment device as a component.
Digital device 38 decodes a data signal received from the CO digital equipment 34 and transmits the decoded data signal to the digital equipment devices 40. The digital device 38 also encodes data signals received from the digital equipment units 40 into a data signal for transmission to the digital equipment 34. Modulation schemes used to communicate between CO 22 and CP 24 may include, but are not limited to, carrierless amplitude/phase modulation (CAP), quadrature amplitude modulation (QAM), Discrete Multi Tone (DMT) or pulse amplitude modulation (PAM), and are commonly known in the art and are not described in detail herein.
FIG. 2 is a simplified illustrative block diagram of a portion of an existing telephony system 20′, which includes a telephone company central office (CO) 22 having POTS switching equipment 28, low pass filter 36 and digital equipment 34, coupled to a customer premises (CP) 24, via a subscriber loop 26, employing multiple channel DSL technology.
With multiple channel DSL, the plurality of digital devices 38 may communicate concurrently with digital equipment 34 employing time-division multiplexing. For convenience, only four digital devices 38 coupled to four PCs 40 are shown. Also, only two telephones 30 and filters 32 are shown. However, any number of either digital devices 38 or telephones 30 could be coupled to subscriber loop 26.
With the system of FIG. 2, any number of the PCs 40 may be concurrently communicating (within their allotted time period and/or allocated band-width) with digital equipment 34 at the CO 22 using time-division multiplexing and/or frequency division multiplexing. Also, one or both of the telephones 30 may be communicating with other telephones (not shown) through POTS switching unit 28 at the same time that the PCs 40 are communicating with digital equipment 34 because the PCs 40 and telephones utilize different logical channels, as described above. Of particular interest is that two people may each be simultaneously using the two telephones 30, such as in a conference call. Because the CP 24 is typically under the ownership and/or control of a single customer, conference calling is acceptable from a convenience and security viewpoint. That is, eavesdropping at the CP 24 is not generally of concern to the CP owner, and if so, the CP owner would be responsible for taking the appropriate safeguards and for implementing any security measures to prevent undesirable eavesdropping at CP 24.
With the advent of multiple channel DSL technology, attempts have been made to couple a plurality of different subscriber loops to a single multiple channel DSL digital equipment unit, thus coupling a plurality of different CPs to a single multiple channel DSL digital equipment unit, such as multiple virtual line (MVL) technology or the like. For convenience, multiple virtual line technology will be referred to as MVL, such use of the phrase MVL is intended to encompass all forms of multiple line technology. FIG. 3 is a simplified illustrative diagram of one such possible system 20″. MVL transceiver unit 60 is similar in functionality to the digital equipment 34 (FIGS. 1 and 2) in that MVL transceiver 60 encodes and decodes data signals which are transmitted to or received from digital devices 38A-38D. However, MVL transceiver 60 may have other advantages and features (which are not described in detail herein because such features and advantages are not relevant to the functioning of the present invention described hereinafter).
Four customer premises 24A-24D are coupled to CO 22 via four different subscriber loops 26A-26D, respectively. For convenience, a single telephone 30A-30D resides in each of the CPs 24A-24B, respectively, and is coupled to POTS switching unit 28 to provide connectivity to the outside communication system. PCs 40A-40D are coupled to digital devices 38A-38D, respectively, and communicate over subscriber loops 26A-26D. Telephones 30A-30D also communicate over subscriber loops 26A-26D, respectively, through filters 32A-32D, respectively. Low pass filters 36A-36D, or POTS splitters in some applications, provide for splitting off the POTS signal to the POTS switching unit 28 and for splitting off the data signals to the MVL transceiver 60.
The application of MVL technology, as illustrated in FIG. 3, has one undesirable aspect that has at least one heretofore unaddressed need. This need arises from the fact that the POTS switching unit 28 at the CO 22, via subscriber connections 26A-26D, concurrently provides service to telephones 30A-30D, respectively. These telephones 30A-30D are electrically coupled to each other through a high impedance path via their respective subscriber loops 26A-26D, and the MVL transceiver 60. The high impedance path is such that when two or more persons are talking on two or more telephones 30A-30D, respectively, the audible interference between the telephones is generally negligible. However, as illustrated in FIG. 4A and FIG. 4B, a small amount of a POTS signal, referred to hereinafter as a leakage signal, may be communicated from one of the telephones onto the other subscriber loops.
For example, a person talking on telephone 30A may be sending/receiving a POTS analog signal over subscriber loop 26A (FIG. 3). Because low pass filter 36A may not be entirely efficient in splitting off the POTS analog signal associated with telephone 30A, some of that POTS signal may be detected on connection 62. This leakage signal may also propagate through low pass filters 36B-36D and may be detected on subscriber loops 26B, 26C and/or 26D. Although the amplitude of the POTS analog signal from telephone 30A is not sufficiently great enough to interfere with analog communications from telephones 30B-30D, this leakage signal from telephone 30A may be nonetheless detectable in some situations.
Moreover, in the above-described illustrative example, the user of telephone 30A at CP 24A typically does not want his telephone conversation detectable by a third party who may have access to subscriber loops 26B-26D. That is, the user of telephone 30A typically does not want their conversation being communicated over subscriber lop 26A to be eavesdropped on. For example, the user of telephone 30A may be a stockbroker or security analyst who may be discussing confidential information. An eavesdropper may desire to eavesdrop on the conversation to gain access to the potentially valuable confidential information. Such an eavesdropper, having access to one of the subscriber loops 26B-26D, could detect the leakage signal with appropriate amplification equipment such that the conversation on telephone 30A could be overheard. Thus, there is an heretofore unaddressed need to prevent a third party eavesdropper from overhearing leakage signals that may exist on subscriber loops which have been coupled into a common multiple virtual line (MVL) transceiver 60.
FIGS. 4A and 4B are simplified illustrative examples of the above-described situation wherein a leakage signal (FIG. 4B) associated with a telephone conversation (FIG. 4A) being communicated across subscriber loop 26A (FIG. 3) may be detectable on subscriber loop 26D. FIG. 4A illustrates the available communication system frequency spectrum 70 for subscriber loop 26A. The POTS channel utilizes a portion of the available frequency spectrum from approximately 0-4 KHz. The conversation of the user of telephone 30A would generate an analog POTS signal 72 as shown in FIG. 4A. (For purposes of conveniently illustrating the various signals shown in FIGS. 4A and 4B, the signal amplitude axis has not been numbered. One skilled in the art will realize that any appropriate axis numbering system could have been employed, and that such a numbering system is not necessary to explain the nature of the leakage signal.) Also shown in FIG. 4A is a data signal 74. Data signal 74 would be a data signal transmitted/received by PC 40A (FIG. 3) over subscriber loop 26A, through digital device 38A and MVL transceiver 60. This data signal occupies a logical channel utilizing a portion of the available communication frequency spectrum between a frequency of F1 and a frequency of F2. (One skilled in the art will appreciate that the actual frequency values F1 and F2 need not be described to explain the nature of the leakage signal.)
FIG. 4B illustrates signals on the available communication system frequency spectrum 76 on subscriber loop 26D (FIG. 3). Data signal 78 is the signal transmitted/received by PC 40D over subscriber loop 26D. Data signal 78 occupies a portion of the available frequency spectrum from a frequency of F3 to F4. (One skilled in the art will appreciate that the frequencies F3 and F4 need not be specified for an understanding of the leakage signal, and that frequencies F3 and F4 may or may not correspond to frequencies F1 and F2 of FIG. 4A depending upon the characteristics of the MVL transceiver 60 and the particular multiplexing scheme employed.) Leakage signal 80 is shown to be present on subscriber loop 26D on the POTS analog channel (0-4 KHz). Leakage signal 80 is associated with the analog POTS signal 72 of FIG. 4A. Leakage signal 80 is seen to be a low amplitude signal, being only a fraction of the amplitude of signal 72 (FIG. 4A) and thus, is seen to be of a sufficiently low amplitude such that leakage signal 80 would not significantly interfere with telephone conversations on subscriber loop 26D (FIG. 3). However, the amplitude of leakage signal 80 may be such that an eavesdropper could detect and amplify leakage signal 80, and thus eavesdrop on the phone conversation on telephone 30A.
Leakage signal 80 arises from the manner in which a plurality of communication connections are coupled to a single communication device, such as the MVL transceiver 60. Each of the communication connections are physically coupled to each other by virtue of their connection to various electrical devices. For example, as illustrated in FIG. 3, subscriber loop 26A is physically coupled to subscriber loop 26D through low pass filter 36A, communication connection 62 and low pass filter 36D. Because of the impedance characteristics associated with the electrical devices which separate subscriber loop 26A and 26D, communication signals associated with telephone conversations on subscriber loop 26A are typically attenuated such that leakage signals associated with telephone conversations on subscriber loop 26A will not substantially interfere with communications occurring on subscriber loop 26D. One skilled in the art will appreciate that leakage signal 80 will have some characteristics which are similar to the well known phenomenon of cross-talk. However, cross-talk is quite different from the leakage signal 80. Cross-talk arises from the inductive or capacitive coupling between two communication connections which are substantially adjacent and parallel to each other. Thus, leakage signal 80 is not considered to be a cross-talk phenomenon.